CS 1101 Programming Methodology http www comp nus

CS 1101: Programming Methodology http: //www. comp. nus. edu. sg/~cs 1101 x/ Aaron Tan

This is Week 12 § Week 10: § Chapter 11: Type Details and Alternate Coding Mechanisms § This week: § Chapter 14: Exception Handling § Chatper 15: Files 2

Writing robust programs (1/3) n Suppose you have this statement int value = std. In. next. Int(); n So far, we are assuming that the user always follows instructions and never enters the wrong data. But what if the user enters the following in response to the above statement? ¨A string (eg: “apple”)? ¨A real number (eg: 12. 3)? 3

Writing robust programs (2/3) n Refer to Wrong. Input. java ¨ User enters a string: An exception is thrown. Enter an integer: apple Exception in thread "main" java. util. Input. Mismatch. Exception at java. util. Scanner. throw. For(Scanner. java: 819) at java. util. Scanner. next(Scanner. java: 1431) at java. util. Scanner. next. Int(Scanner. java: 2040) at java. util. Scanner. next. Int(Scanner. java: 2000) at Wrong. Input. main(Wrong. Input. java: 9) ¨ User enters real number: Enter an integer: 12. 34 Exception in thread "main" java. util. Input. Mismatch. Exception at java. util. Scanner. throw. For(Scanner. java: 819) at java. util. Scanner. next(Scanner. java: 1431) at java. util. Scanner. next. Int(Scanner. java: 2040) at java. util. Scanner. next. Int(Scanner. java: 2000) at Wrong. Input. main(Wrong. Input. java: 9) 4

Writing robust programs (3/3) n An exception is thrown when an error occurs. If the exception is not caught, the program crashes. n We would like to catch the exception so that the program does not crash. n Let’s go on to Chapter 14 to see how to handle exceptions. 5

Using throws to postpone catch n n Refer to Student. List 2. java and Student. List 2 Driver. java For checked exceptions, if you do not have a try-catch block, then you must add ‘throws’ at the heading of the method, or it will not compile. 6

The finally Block n Sometimes there are situations where we need to take certain actions regardless of whether an exception is thrown or not. For example, we need some “cleanup” code, like closing a file. Use a finally block. n Refer to Write. To. File. java n 7

Summary: try-catch block Exception try { <t-stmt-1> <t-stmt-2> <t-stmt-3> <t-stmt-4>. . . <t-stmt-n> No Exception try { Assume <t-stmt-3> throws an exception. Remaining statements in the try block is skipped. } catch (Exception e) { <c-stmt-1> Statements in the. . . catch block are executed. <c-stmt-m> } And the execution <next stmt> continues to the <t-stmt-1> <t-stmt-2> <t-stmt-3> <t-stmt-4>. . . <t-stmt-n> All statements in the try block are executed. } catch (Exception e) { <c-stmt-1> Statements in the. . . catch block are skipped. <c-stmt-m> } <next stmt> next statement Ack: Thomas Wu 8

Summary: multiple catch blocks Exception try { <t-stmt-1> <t-stmt-2> <t-stmt-3> <t-stmt-4>. . . <t-stmt-n> No Exception Assume <t-stmt-3> throws an exception and <catch-block-3> is the matching block. try { <t-stmt-1> <t-stmt-2> <t-stmt-3> <t-stmt-4>. . . <t-stmt-n> Remaining statements in the try block is skipped. } All statements in the try block are executed and throw no exceptions. } <catch-block-1> <catch-block-2> <catch-block-3>. . . <catch-block-m> } <next stmt> Ack: Thomas Wu Statements in the matching catch block are executed. <catch-block-1> <catch-block-2> <catch-block-3>. . . <catch-block-m> All catch blocks are skipped. } <next stmt> 9

Summary: with finally block Exception No Exception Assume <t-stmt-i> try { throws an exception <t-stmt-1> and <catch-block-i> is. . . the matching block. <t-stmt-i>. . . <t-stmt-n> } <catch-block-1>. . . <catch-block-i>. . . <catch-block-m> try { <t-stmt-1>. . . <t-stmt-i>. . . <t-stmt-n> } <catch-block-1>. . . <catch-block-i>. . . <catch-block-m> } finally {. . . } <next stmt> Ack: Thomas Wu finally block is executed. 10

Summary: Hierarchy of exceptions n There are over 60 classes in the hierarchy. Here are just some. Ack: Thomas Wu 11

Handling files in UNIX § We have seen file processing in Java. § Let me introduce some features in UNIX that handle files (we did it in our intro lab, if you still remember!) § This is not part of Java. 12

UNIX File Input Redirection n <: To redirect input from a file $ java My. Program < input. File The text file input. File contains the input data. $ java My. Sum Enter 3 integers: 4 2 3 Sum = 9 $ cat sum. in 4 2 3 $ java My. Sum < sum. in Enter 3 integers: Sum = 9 13

Example n My. Sum. java import java. util. *; class My. Sum { public static void main(String[] args) { Scanner std. In = new Scanner(System. in); System. out. print("Enter 3 integers: "); int a = std. In. next. Int(); int b = std. In. next. Int(); int c = std. In. next. Int(); int sum = a + b + c; System. out. println("Sum = " + sum); } } 14

UNIX File Output Redirection n >: To redirect output to a file (>>: to append to a file) $ java My. Program > output. File The text file output. File contains the output data. $ java My. Sum Enter 3 integers: 4 2 3 Sum = 9 $ java My. Sum > sum. out 4 2 3 $ cat sum. out Enter 3 integers: Sum = 9 15

Announcement/Reminder § Lab #5 § Deadline: 5 November (Wednesday), 2359 hr. § PE § Please refer to website http: //www. comp. nus. edu. sg/~cs 1101 x/3_ca/pe. html § Consultation @ my office (COM 1 -03 -12) § Wednesday (5 Nov) 3 – 6 pm 16

This is Week 12 § Next week (last week!) § Chapter 12: Aggregation, Composition, and Inheritance 17

End of file 18